1. Field of the Invention
The present invention relates to a radar device capable of transmitting a radar wave and receiving a reflected radar wave (as an arrival wave), and obtaining target information regarding a target position, a traveling (moving) speed, and a target azimuth (or the direction) of the target object based on received signals, corresponding to the reflected radar wave, obtained through an array antenna.
2. Description of the Related Art
There are various types of known conventional radar devices, for example, one of which is mounted to one's own vehicle (hereinafter, referred to as the “driver's vehicle”), and generates and transmits a transmission radar wave as an observation signal toward the forward direction of the driver's vehicle. The on-vehicle radar device receives a reflected radar wave (or an arrival wave) reflected by a target object such as a preceding vehicle. This preceding vehicle is present or traveling in front of the driver's vehicle. The on-vehicle radar device calculates a target distance, a target azimuth (or a target angle), and a relative traveling speed of the target object based on the reflected radar wave. That is, the target distance of the target object as a preceding vehicle is a distance between the driver's vehicle and the target object which is present in front of the driver's vehicle. The target azimuth is an azimuth of the target object observed from the position of the driver's vehicle. The relative speed is a traveling speed of the target object in consideration with the traveling speed of the driver's vehicle.
There is a FMCW (Frequency Modulated Continuous Wave) radar device as an on-vehicle radar device. For example, there are related-art documents showing such a FMCW radar device, for example, Japanese patent laid open publication No. JP 2006-284182 and No. JP 2006-300720 have disclosed such a FMCW radar device.
As shown by the solid line at the upper part in FIG. 10, the FMCW radar device transmits a radar wave as a transmission signal (or a sending signal) Ss, and then receives a reflected radar wave (or an arrival wave) reflected by the target object such as a preceding vehicle, as shown in FIG. 11A. The transmission signal Ss is obtained by frequency-modulation using an modulation signal on a triangle wave so that the frequency of the transmission signal Ss is linearly and gradually increased, and decreased in the course of time (see the upper side in FIG. 10).
At this time, as designated by the dotted line at the upper side of FIG. 10, the received signal Sr corresponding to the reflected radar wave and received through the array antenna is delayed in time from the transmission signal Ss by the time tr. This time tr corresponds to the time required for the radar wave to travel between the target object and the driver's vehicle, namely, corresponds to the distance between the target object and the driver's vehicle. The received signal Sr is doppler-shifted by the frequency fd toward the frequency-down direction corresponding to the relative speed between the target object and the driver's vehicle.
The FMCW radar device mixes the received signal Sr with the transmission signal Ss to produce a beat signal BT (see the bottom side in FIG. 10). The FMCW radar device calculates a target distance D and a relative speed V between the driver's vehicle and the target object based on a frequency fb1 and a frequency fb2 of the beat signal BT according to the following equations (1) to (4). The frequency fb1 of the beat signal BT is a frequency during a upward section (or a rising section) in which the frequency of the transmission signal Ss is increased, and the frequency fb2 of the beat signal BT is a frequency during a downward section (or a falling section) in which the frequency of the transmission signal Ss is decreased.
                              D          =                                    c              ·              fr                                                      4                ·                fm                ·                Δ                            ⁢                                                          ⁢              f                                      ,                            (        1        )                                          V          =                                    c              ·              fd                                                      2                ·                f                            ⁢                                                          ⁢              0                                      ,                            (        2        )                                          fd          =                                                    fb                ⁢                                                                  ⁢                1                            -                              fb                ⁢                                                                  ⁢                2                                      2                          ,                                  ⁢        and                            (        3        )                                          fr          =                                                    fb                ⁢                                                                  ⁢                1                            +                              fb                ⁢                                                                  ⁢                2                                      2                          ,                            (        4        )            where “c” designates a propagation speed of a transmission signal Ss such as a radar (or radio) wave, “fm” denotes a modulated frequency of the transmission signal Ss, “Δf” denotes a width in fluctuation of the transmission signal Ss, and “f0” designates a central frequency of the transmission signal Ss.
That is, the FMCW radar device performs Fourier transformation of the beat signal BT, and then performs Frequency analysis in order to specify the frequency fb1 of the reflected wave component of the beat signal BT in the upward section, and the frequency fb2 of the reflected wave component of the beat signal BT in the downward section.
The FMCW radar device then obtains the target distance D to the target object which is present or traveling in the front area of the driver's vehicle, and the relative speed V between the target object and the driver's vehicle based on the calculated frequencies fb1 and fb2.
The FMCW radar device calculates the azimuth of the target object observed from the driver's vehicle based on the reflected radar wave received by each of antenna elements in the array antenna as a receiving antenna, where the reflected wave has a phase difference corresponding to its arrival direction or coming direction. There has been known a method of obtaining the direction of the target using the array antenna composed of a plurality of antenna elements. In the method, an auto-correlation matrix of the received signal obtained through each of the antenna elements is firstly generated, an angle spectrum is generated based on the auto-correlation matrix, and the angle spectrum is analyzed in order to obtain the azimuth of the target. For example, there have been known MUSIC (MUltiple SIgnal Classification) method, DBF (Digital Beam Forming) method, and CAPON method as the direction calculation method to calculate the azimuth of the target.
A description will now be given of the explanation of the MUSIC method which is one of well-known methods to calculate the arrival direction (or coming direction) of a reflected wave. In the following explanation, the array antenna is a linear antenna composed of “k” antenna elements which is arranged in line at constant interval, where “k” is an integer. This type of the array antenna will be referred to as the “linear array antenna”.
At first, Fourier transformation is performed for a beat signal BT, which corresponds to each of the antenna elements that forms the array antenna. A received vector X expressed by the following equation (5) is obtained by arranging Fourier transformed values at the peak frequency of each of the beat signals BT corresponding to the antenna elements of the array antenna. Next, an auto-correlation matrix Rxx with k rows and k columns expressed by the following equation (6) is obtained using the received vector X.X=[x1, x2, . . . , xK]T  (5), andRxx=XXH  (6)where, an element xk (k=1, . . . , and K) of the received vector X corresponds to the Fourier transformed value (as complex numbers) of k-th antenna element at the peak frequency which is commonly appeared in each of the K antenna elements. The value T in the above equation (5) designates a vector transpose, and the value H designates a complex conjugate transpose.
Because the peak frequency indicates the frequency of the reflected radar wave in the ideal condition where the receive signal received by each of the antenna elements does not contain any noise, the peak frequency is one of the above frequencies fb1 and fb2.
In general, the received vector X expressed by the equation (5) is obtained by performing Fourier transformation of the best signals BT in each of an upward section and a downward section, obtaining the peak frequency every the upward section and the downward section, and then arranging the Fourier transformed value of each of the antenna elements at the pear frequency.
Next, the azimuth of the target object, at which the transmission wave as the radar wave is reflected, is calculated by the following procedure using the auto-correlation matrix of the received vector X which is generated every the upward section and the downward section.
Specifically, eigenvalues λ1, . . . , and λK (where, λ1, ≧λ2, ≧ . . . λK) of the auto-correlation matrix Rxx are obtained. The number M of arrival waves is estimated based on the number of the eigenvalues λK which are greater than a threshold value λth corresponding to a thermal noise (Johnson-Nyquist noise) power. Further, the eigenvalue vectors eM+1, . . . , and eK corresponding to (K-M) eigenvalues λM+1, . . . , λK which are not more than the thermal noise power are calculated.
The MUSIC spectrum expressed by the following evaluation function PMU(θ) expressed by the following equation (8) as the angle spectrum is then obtained from a noise eigenvalue vector EN expressed by the following equation (7), and a complex response to a target azimuth θ, namely, a steering vector a(θ), where the noise eigenvalue vector EN is composed of the eigenvalue vectors eM+1, . . . , and ek corresponds to (K-M) eigenvalues λM+1, . . . , and λK is not more than the thermal noise power.
                                          E            N                    =                      (                                          e                                  M                  +                  1                                            ,                              e                                  M                  +                  2                                            ,              …              ⁢                                                          ,                              e                K                                      )                          ,                                  ⁢        and                            (        7        )                                                      P            MU                    ⁡                      (            θ            )                          =                                                                              a                  H                                ⁡                                  (                  θ                  )                                            ⁢                              a                ⁡                                  (                  θ                  )                                                                                                      a                  H                                ⁡                                  (                  θ                  )                                            ⁢                              E                N                            ⁢                              E                N                H                            ⁢                              a                ⁡                                  (                  θ                  )                                                              .                                    (        8        )            
As shown in FIG. 11B, because the MUSIC spectrum expressed by the evaluation function PMU(θ) expressed by the equation (8) has a sharp spectrum when the azimuth θ is coincided with the arrival direction of the arrival wave, the azimuth θ1, . . . , θM of the arrival wave, namely the azimuth of the target, by which the transmission wave is reflected, can be obtained by extracting the peak (null point) of the MUSIC spectrum.
That is, the conventional radar device obtains the peak frequency based on the power spectrum of the beat signal BT every section such as the upward section and the downward section, and the azimuth θ1, . . . , θM of the arrival wave (as the reflected radar wave) of the peak frequency is obtained from the peak of the MUSIC spectrum in order to obtain the azimuth θ of the arrival wave from the target object (or the reflected wave reflected by the target).
It is necessary to obtain the azimuth of the target object every section such as the upward section and the downward section because a plurality of perk frequencies are detected in the power spectrum of the beat signal BT in each of the upward section and the downward section when a plurality of preceding vehicles are present or traveling in front of the driver's vehicle, and the array antennal mounted onto the driver's vehicle receives a plurality of the reflected radar waves as arrival waves reflected by those preceding vehicles.
Presence of a plurality of peak frequencies in each section makes it difficult to accurately determine which combination of peak frequencies indicates the combination of the frequencies fb1 and fb2. In order to solve this difficulty, the conventional radar device obtains the azimuth θ of each of the peak frequencies in each section such as the upward section and the downward section, and then specifies the peak frequency in the upward section and the peak frequency in the downward section having the same azimuth θ thereof as the combination of the frequencies fb1 and fb2. The conventional radar device obtains the target distance D of the target object, the relative speed between the target object and the driver's vehicle, and the target azimuth θ of the target object based on the combination of the specified peak frequencies.
By the way, as shown in FIG. 12A, the beat signal BT also contains, in addition to the reflected radar wave reflected by the target vehicle, for example, transmitted radar waves from a radar device mounted on the front of a vehicle which is traveling on an opposite lane of the road, and/or transmitted radar waves from a radar device mounted on the back of a vehicle that is traveling on the same lane of the road in front of the driver's vehicle, as well as noise from various sources.
The conventional radar device calculates the auto-correlation matrix Rxx based on the beat signal BT obtained every each cycle, (each cycle has a modulation period (1/fm) of the transmission signal) by the above method, and calculates an equivalent average of the auto-correlation matrices Rxx in a plurality of continuous cycles in time in order to obtain the section average correlation matrix R0. The conventional radar device then performs the above method to obtain the MUSIC spectrum based on the section average correlation matrix R0 expressed by the following equation (9), and then calculates the azimuth of the target object based on the MUSIC spectrum. This procedure can calculate the target azimuth θ with less influence of noise.
                              R          ⁢                                          ⁢          0                =                              1            SNN                    ⁢                                    ∑                              i                =                1                            SNN                        ⁢                                                  ⁢                                          Rxx                ⁡                                  (                  i                  )                                            .                                                          (        9        )            
The section average correlation matrix R0 is calculated by the equation (9) using the equivalent average of the auto-correlation matrices Rxx for SNN cycles. In the equation (9), Rxx(i) designates the auto-correlation matrix Rxx of the i-th cycle in the auto-correlation matrices Rxx to be used for the equivalent average.
As described above, obtaining the section average correlation matrix R0 can suppress the influence of noise, and it is thereby possible to certainly obtain the target azimuth θ of the target object with higher accuracy when compared with the case of obtaining the target azimuth θ of the target object based on the auto-correlation matrix Rxx every cycle.
When the target position and traveling speed of the target vehicle are calculated based on the power spectrum in each cycle, calculating them using the peak frequencies obtained from the power spectrum of the beat signals BT can be easily affected by noise.
In order to eliminate the influence from noise, the conventional radar device performs the equivalent average of the power spectrum of the beat signal BT in a plurality of continuous cycles in time, and obtain the peak frequency from the averaged power spectrum, and then obtains the target position and traveling speed of the target object based on the peak frequencies in order to suppress the influence of noise.
However, the conventional radar device having the above structure makes it difficult to obtain the current position, traveling speed (or a target speed), and azimuth of a target object with high accuracy because of performing the equivalent average of the auto-correlation matrix and the power spectrums in a plurality of cycles. In other words, the conventional radar device has a limitation to obtain the target current position, the target traveling speed, and the target azimuth with high accuracy.